how to get data from gridview footer to display in textbox outside the gridview?

Question

hi,

Is anyone know how to get data from gridview footer and show it in textbox outside the gridview. Here I give the example, I make a total price in gridview footer. Then I want to call back the total and show the total in a textbox. The problem is I always get this error 

Object reference not set to an instance of an object.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Line 68: 
Line 69:             Dim c As Integer = 0
Line 70:             Dim j As String = CType(GridView3.Rows(c).Cells(3).FindControl("Total:"), TextBox).Text
Line 71:             TextBox2.Text = j
Line 72: 

My coding is like this

 Protected Sub GridView3_RowDataBound(ByVal sender As Object, ByVal e As GridViewRowEventArgs) Handles GridView3.RowDataBound
        If e.Row.RowType = DataControlRowType.DataRow Then
            Dim rowTotal As Decimal = Convert.ToDecimal(DataBinder.Eval(e.Row.DataItem, "SPA"))
            subTotal = subTotal + rowTotal
        End If

        If e.Row.RowType = DataControlRowType.Footer Then
            e.Row.Cells(3).Text = "Total:"
            e.Row.Cells(4).Text = subTotal.ToString("c")

            Dim c As Integer = 0
            Dim j As String = CType(GridView3.Rows(c).Cells(3).FindControl("Total:"), TextBox).Text
            TextBox2.Text = j

        End If
    End Sub

I really hope that someone could help me with this....thank u..